Along with the growth of people's demands for liquid fuel based on petroleum and the abatement of exploitable oil reserves, producing usable liquid fuel from solid fuel, such as coal, attracts more and more attention.
At present, coal is mainly used for power generation, coking and other industries or civil fuel. In some filed such as fuel field and power field, low-sulfur high quality or specific coals are needed. However, a great part of coal reserves cannot meet these requirements. China is abundant in coal resources, but only half of them are of high quality, and the rest are lignite or other kinds of coals. Low-rank coal including lignite represents about up to 40% of the total Chinese coal reserves, especially in east Inner-Mongolia, this value reaches up to 70% or more. Reasonable exploitation and utilization of lignite becomes a big issue, so the exploitation of lignite remains very limited until now. For instance, in China, total coal production arrives at 2151.31 million tons in 2005, wherein high quality coal constitutes 65%, but lignite, which has a comparatively higher reserve, achieves a production of 97.64 million tons and represents only 4.5% of the total coal production.
The utilization of low-rank coal such as lignite raises the following problems: (1) when high quality coal is exploited, the intergrowth and associated low quality coal would not be mined due to its low market value, resulting in a considerable waste of coal resources. (2) because of its high moisture, high ash, high volatile matter content and low heating value, lignite is usually regarded as a low quality steam coal; in addition, due to its fragileness and susceptibility to differentiation and spontaneous combustion, it is very difficult for long-haul transportation, making lignite only suited for local utilization. Hence, some alternative ways to make use of low-rank coals should be developed so as to regulate the structure of coal exploitation and consumption, thus achieve unification of the structure of coal resource, production and consumption and also reduce the overall consumption intensity of the coal resource.
Among many ways of using above-mentioned coals, substantially high quality coal has become feedstock for fuel and power industries with low utilization efficiency and heavy pollution. China's immense coal-based power industry has been operated under efficiency around 30 to 35% for a long time, which causes serious pollution problem and significant waste of resource. Along with power industry turning to nuclear power, wind and solar energy, greater efficiency for coal use becomes very important to the sustainable development in the future for China, even for the world. Therefore, CTL (Coal to Liquid) will be one of the most significant technologies for manufacturing oil products, which are in shortage but crucial for the human society's development. Besides, biomass can also be treated through the same process to yield oil as coal liquefaction; it can be a potential and effective solution to human's sustainable development in long term after coal is exhausted.
Generally speaking, there are three different processes to produce oil by coal liquefaction: partial liquefaction, which partially transforms coal to tar by carbonization; direct liquefaction, that completely converts coal into oil directly by hydrogenation; indirect liquefaction, that firstly turns coal into syngas (CO+H2), then produces high quality oil from syngas via Fischer-Tropsch (F-T) reaction. These three methods have been industrialized from last century and became German's main oil producing source during the 2nd World War. Early period of coal liquefaction research for large-scale industrialization had lasted till 1960's, thereafter the discovery and intensive exploitation of petroleum halted it. However, scientific research in laboratory continued and a particular coal conversion domain appeared. In 1950's, based on coal gasification and Fischer-Tropsch technology, South Africa realized an industrial scale CTL plant with indirect liquefaction process, SASOL's CTL plant.
The world petroleum crisis in 1970's started a new era for coal liquefaction research. Thanks to the technique improvement in petroleum processing, the research work obtained good progress at higher levels from 1970 to 1990, for instance, modern pressurized coal gasification technology “entrained flow bed coal gasification” came out, became the foundation of large-scale coal gasification technology and accelerated the syngas chemistry's industrialization. Industrial development of large-scale manufacturing of ammonia and methanol from coal was just based on efficient and massive coal gasification. Meanwhile, new techniques for direct coal liquefaction by hydrogenation working under milder conditions appeared, like direct liquefaction techniques of “HTI” from US and “Nedol” from Japan; also, large-scale Fischer-Tropsch techniques emerged, such as fixed fluidized-bed and three-phases suspended bed (SASOL). The most remarkable progress in this period was from SASOL, which increased its capacity of indirect coal liquefaction from 1 million tons/year to 6 million tons/year.
Due to the severe operation conditions for direct coal liquefaction by hydrogenation, it is a huge challenge for equipments' manufacture, operation safety and products' quality, so this technology has stayed on pilot experiments and demonstration operating level. The technology of coal's low temperature carbonization developed at the same time can reach 1 million to 3 million tons/year by one single train, but its industrialization procedure was prevented by low oil production rate (no more than 10%), high cost of tar treatment, low valuable semi-coke and serious environmental pollutions. In addition, petroleum price being unstable and not high enough for a long time also made direct coal liquefaction's development stagnant.
Recently, successful researches have boosted diversified worldwide CTL's development into industry level; besides, oil supply becomes shorter and shorter because of petroleum's great consumption and deficient storage. Industrializing CTL technology will be more and more compelling in the next 20 years.
Looking into modern CTL technology, large-scale coal carbonization's biggest challenges is to develop efficient and environment-friendly processes, to effectively manage and make use of atmospheric carbonization gas (including flue gas and combustible gas). Direct liquefaction by hydrogenation still faces some issues in equipments' fabrication, safe and stable operation, and low-quality diesel oil products etc.; for indirect liquefaction, although we surpass the obstacles of scale-up, safety and environment protection, the problem that too much coal is dedicated to per ton oil products remains because of the low heating value of gasification feedstock.
The invention in this article proposes, based on industrialized indirect coal liquefaction process, to partially liquefy carbonaceous solid combustible feedstock, especially low-rank coal and biomass, by moderate hydrogenation before entering to large-scale gasification stage, so that the light oil components of the feedstock are extracted and the highly condensed heavy oil components with great heating value are gasified to produce syngas. A portion of syngas is utilized to produce hydrogen (H2) necessary for the process and the rest is treated by multistage liquefying of F-T synthesis oil technique. The above-mentioned staged liquefaction effectively combined the direct coal liquefaction by hydrogenation with Fischer-Tropsch reaction, thus achieving the efficiency optimization for the entire process of producing high quality oil from coal. The overall efficiency of this process is approved much higher than any other clean coal utilization method; besides, it yields excellent liquid fuel and chemical materials.